feat(notif): NotificationOutboxActor + CentralAuditWriter wired (#23 M4)

M4 Bundle B (B1) — add the central-only ICentralAuditWriter implementation
and inject it into NotificationOutboxActor so subsequent tasks (B2/B3) can
route attempt + terminal lifecycle events through the direct-write audit path.

- CentralAuditWriter: thin wrapper around IAuditLogRepository.InsertIfNotExistsAsync;
  scope-per-call (matches AuditLogIngestActor / NotificationOutboxActor pattern);
  stamps IngestedAtUtc; swallows all internal failures (alog.md §13).
- Registered as a singleton in AddAuditLog.
- NotificationOutboxActor ctor takes ICentralAuditWriter (validated non-null).
- Host wiring resolves the writer once from the root provider and passes it
  into the singleton's Props.Create call.
- Existing TestKit fixtures updated with a NoOpCentralAuditWriter helper so
  tests that don't exercise audit emission still compile and pass.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -275,11 +275,18 @@ akka {{
             .GetRequiredService<IOptions<ScadaLink.NotificationOutbox.NotificationOutboxOptions>>().Value;
         var outboxLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
             .CreateLogger<ScadaLink.NotificationOutbox.NotificationOutboxActor>();
+        // M4 Bundle B: central direct-write audit writer for dispatcher attempt
+        // + terminal events. Resolved once from the root provider — the writer
+        // is a singleton and stateless, opening per-call DI scopes internally
+        // to resolve the scoped IAuditLogRepository.
+        var outboxAuditWriter = _serviceProvider
+            .GetRequiredService<ScadaLink.Commons.Interfaces.Services.ICentralAuditWriter>();
 
         var outboxSingletonProps = ClusterSingletonManager.Props(
             singletonProps: Props.Create(() => new ScadaLink.NotificationOutbox.NotificationOutboxActor(
                 _serviceProvider,
                 outboxOptions,
+                outboxAuditWriter,
                 outboxLogger)),
             terminationMessage: PoisonPill.Instance,
             settings: ClusterSingletonManagerSettings.Create(_actorSystem!)